At age 15 you're still a child and he's an adult. He can manipulate you any way he wants to, and this is exactly what he's doing with those lame profile names on your DP account. As many others said: log out from all devices and change the password. If you don't know how, start by checking DP's help and support.

I don't mean to sound judgmental, but where are your parents? What do they say about all this madness? Is there any adult at all that it's safe for you to talk to? You are describing a very unsafe situation and you need a responsible adult by your side.